Hi--

>Those were great lessons in photography on the net.

Hmmm, thanks for the comment! (Uh, which lessons.....? ;-)

>As for me, I'm a research assistant with Texas A&M University (Mechanical
>Engineering Dept.). I recently ran into a problem while purchasing a lens
>for my research work. The lens is used for observing fractured specimens
>in the lab (used along with a video camera). The lens, being used, has
>specifications as 18-108 mm, f2.5 ( TV Zoom lens). It gives a max.
>magnification of 65 times - on my monitor. The object under observation
>is 1 ft from the lens.
>
>I need a lens that would give me a magnification of about a 120 times.
>I needed your advice on what type of lens (specification etc.) should I
>go for. Also given the focal length and f-stop (as in my case) how can we
>determine the max. magnification of a lens?

I might not be the right one to ask about this, but... If you can change
the subject to lens distance, you can probably pick up the 2X magnification
by closer focus using extension tubes or by using an achromat on the front of the 18-108 (with enough light to be using the lens at middle to smaller apertures, the sharpness should not be noticeably worsened when using the achromat). Nikon makes two strengths in two sizes (52 and 62mm thread,
which could be adapted, even if the front thread of the zoom is up to
maybe 82mm [vignetting doesn't generally occur in close-ups when using achromats that are considerably smaller than the lens front]).

>As I'm badly in need of an expert opinion before I make a purchase, your
>advice will be of great help. Thanks.
>Awaiting your reply,
>Regards,
>Vikas.
ksvp003@tamuk.edu (Pillai Vikas)

David Jacobson is more of a theoretical technical expert (I lean toward
the pragmatic approach to problems) - maybe he can help, if the above is
not useful.
David Ruether